Ultrastructure and junctional permeability of microvessel endothelia will be studied in rat omentum in situ and in explants. Cultures of microvessel endothelium also will be studied ultrastructurally and physiologically, to determine types of junctions present and their abilities to transfer small molecules from cell to cell. Junctional transfer between endothelial cells and pericytes and/or smooth muscle will be studied as well. Endothelial-smooth muscle cultures will be used as a model for investigating the physiological role of endothelial-smooth muscle junctions.